A flow rate measuring device of recent years is adapted to calculate a measured flow rate by A/D converting sensor signal values from various sensors (such as pressure and temperature sensors) for identifying a flow rate, and then operating the digitized sensor signal values with software using a CPU.
The measured flow rate obtained in this manner has of course a digital value; however, depending on a user, there is a demand to obtain the measured flow rate as an analog signal.
In order to convert the digital value into the analog signal to respond to such a demand, a D/A converter is used.
Among many D/A converters, one that can be easily and inexpensively configured using a pulse width modulation (PWM) output function normally included in a CPU is a PWM type D/A converter.
The CPU can output a PWM signal having a duty ratio corresponding to a digital value indicating a measured flow rate. Also, the PWM type D/A converter is one that smooths the PWM signal through a lowpass filter to convert to an analog signal.
However, this method has the following problems.
A first problem is one between residual ripple and response speed occurring when passing the PWM signal through the lowpass filter. When reducing the ripple to increase the time constant of the lowpass filter in order to stabilize an analog signal value, a problem of deteriorating the response speed to prevent a change in measured flow rate from being immediately reflected in a change in analog signal value occurs. Between the reduction in ripple and the improvement in response speed, there is a tradeoff relationship, and it has been considered that simultaneously achieving the both is difficult.
A second problem is that even though a PWM output voltage from the CPU changes between a digital power supply voltage and a digital common voltage, the digital power supply voltage and the digital common voltage are low in voltage accuracy and easily changed by the effect of noise or the like. Since the analog signal is obtained by smoothing the PWM signal having the unstable high level and low level voltages, the value of the analog signal also follows the PWM signal and is thereby destabilized, which serves as a bottleneck to make it difficult to obtain a highly accurate analog output value.
In addition, the above-described two problems become particularly significant in a flow rate measuring device used for applications such as a semiconductor process of recent years. This is because the flow rate measuring device used for such applications requires accuracy necessary for measuring a minute flow rate or a change in minute flow rate, and also requires high response speed in order to increase throughput.